Hot Rod Deluxe Bias Ramping up ... all by itself.

[Mick Bailey]

I explained to the customer the possible solution and he was happy for me to proceed given that it substantially reduces the repair cost. I didn't even need to unplug the board other than 1 connector. The idea isn't original, though the custom cutter is my own.

As far as removing material goes, here's another job I have in right now that needs material removing. If not, then a new board would be the alternative. Fender boards are pretty expensive in the UK.

[Enzo]

I wouldn't hesitate a moment to grind away the carbon on that board and save it. I would feel no need to ask the customer first, it is a common part of repair.

[christarak]

I explained to the customer the possible solution and he was happy for me to proceed given that it substantially reduces the repair cost. I didn't even need to unplug the board other than 1 connector. The idea isn't original, though the custom cutter is my own.

As far as removing material goes, here's another job I have in right now that needs material removing. If not, then a new board would be the alternative. Fender boards are pretty expensive in the UK.

[ATTACH=CONFIG]43633[/ATTACH]

I see what you mean Mick, especially with that photo. I am so glad I have joined this Forum. The experience of my fellow members have given me the confidence to try new approaches.

[christarak]

I'm not sure if anyone has raised the possibility of oscillation yet?

I really appreciate your comments, g1. I'm a follower of you and Enzo, and feel humbled to receive support from both.

I had not thought of this possibility, g1. Where and how would be the best place for me to test for oscillation? (Tools at hand: digital oscilloscope, signal generator, multimeters, bias probes, bulb current limiter .. and other stuff)

[nosaj]

I really appreciate your comments, g1. I'm a follower of you and Enzo, and feel humbled to receive support from both.

I had not thought of this possibility, g1. Where and how would be the best place for me to test for oscillation? (Tools at hand: digital oscilloscope, signal generator, multimeters, bias probes, bulb current limiter .. and other stuff)

What about a simple AM radio?
Some circuits may rudely oscillate at an RF frequency while still doing a reasonable job of passing audio. The oscillation is more or less continuous, and may cause distortion, or a vague foginess in the sound. Sometimes there are no audible artifacts. These can often be detected with an AM radio or TV set in the same room. Often one or two TV channels are totally wiped out even though the TV is connected to cable and recieving a strong signal. Often moving parts around will change the affected channel, and help lead to the offending circuit. These are the easy ones.

nosaj

[Chuck H]

If your scope has a high enough frequency rating you may be able to detect a high frequency oscillation if it's present. That was a good call on g1's part and worth checking IMO.

The other thing that hasn't been addressed much is your report of an oily substance on the board. Unless something was spilled in the amp this could be indicative of a power supply filter/decoupling cap failure. The HRD has an unusual power supply circuit that combines same phase stages on the same decoupling cap node. A failure in one of these caps could therefor cause an oscillation that is ramping up the current as it is amplified. In fact some early HR-- model amps had a bit of a problem with poor quality filter caps and would oscillate after only a couple of years of use. Replacing the power supply caps was the fix. I never experienced this first hand, but I've paid attention here on the forum and the phenomenon is real.

[g1]

I really appreciate your comments, g1. I'm a follower of you and Enzo, and feel humbled to receive support from both.

And I'm humbled you would mention me alongside Enzo, I'm pretty sure I'm not in his league though.

I had not thought of this possibility, g1. Where and how would be the best place for me to test for oscillation? (Tools at hand: digital oscilloscope, signal generator, multimeters, bias probes, bulb current limiter .. and other stuff)

The AM radio trick Nosaj mentioned is a nice quick check.
As for looking at the scope, you can see an example in this post, although the oscillation is riding on a signal. With the amp idling and no signal, you would see the fuzzies riding on a flat line. http://music-electronics-forum.com/t44335/#post455720

Also, like Chuck said, the spill is worth revisiting. If it is something oily, wiping up may not be enough. Something like isopropyl alcohol should be used to get red of the (possibly conductive) residue.

[Justin Thomas]

Oily Residue makes me think an electrlytic cap 'sploded itself inside. It's one thing when the goo leaks out slowly and has a chance to dry out. It's another when a cap blasts its guts all over the place like a firecracker.

Or someone hosed the whole board with DeOxit or WD40... it's happened before. Either way, I'd clean it up...

Justin

[Chuck H]

I would think a blown up or even a vigorously expelled filter cap would make itself obvious. But I've seen a lot of strange things and I don't even do repairs regularly. I figure that a even a slow leaking cap could oil the whole interior if the leakage gets warm enough to vaporize somewhat. Anyway... A bad or caps should only cause oscillation and therefor an increase in current if the preamp is engaged. So if the knobs are all zeroed, which I do for bias checking and adjustments, this shouldn't be the case unless there is some source for positive feedback after the preamp. Since the NFB loop has been eliminated as a potential cause and other issues like the bias supply have also been eliminated I would probably be looking for a ground fault. I might even signal test the amp in short intervals to get an idea of what all the controls are doing or not doing to narrow the search.

[Justin Thomas]

Maybe somebody already tried to destroy the evidence already...
As in, clean the shrapnel, leave the blood...

Justin

[Enzo]

I'm pretty sure I'm not in his league though.

C'mon g1, we're both in the North American League, Eastern Conference.

[g1]

Hmmm. If you put it that way. Where does the western conference end?

[Enzo]

Second place...


Tha-dump.



Banff maybe?

[christarak]

Oily Residue makes me think an electrlytic cap 'sploded itself inside. It's one thing when the goo leaks out slowly and has a chance to dry out. It's another when a cap blasts its guts all over the place like a firecracker.

Or someone hosed the whole board with DeOxit or WD40... it's happened before. Either way, I'd clean it up...

Justin

That's exactly what it look like Justin: WD40. I washed the board down with a white spirit (we call it methylated spirits in Australia) and the sprayed with Circuit Board Cleaner and compressed air to dry.
But ... I haven't changed the filter caps.

[christarak]

What about a simple AM radio?
Some circuits may rudely oscillate at an RF frequency while still doing a reasonable job of passing audio. The oscillation is more or less continuous, and may cause distortion, or a vague foginess in the sound. Sometimes there are no audible artifacts. These can often be detected with an AM radio or TV set in the same room. Often one or two TV channels are totally wiped out even though the TV is connected to cable and recieving a strong signal. Often moving parts around will change the affected channel, and help lead to the offending circuit. These are the easy ones.

nosaj

I like the AM radio suggestion. The weird thing is...the HRD sounds fine with a guitar plugged in. No fog. No crazy overtones or distortion. It's bias current escalates. Having said that, I will see if there is an AM radio in the house.